1. Field of the Art
This invention relates generally to an endoscope for use in medical fields, and more particularly to an endoscopic observation system having an objective lens of a variable observation distance in its optical image pickup system.
2. Prior Art
Generally, the endoscopes, which are designed for introduction into a body cavity of a patient for medical examination or observation purposes, are largely constituted by a manipulating head assembly to be gripped and manipulated by an operator, and an elongated insertion instrument which is extended out from the head assembly for insertion into a body cavity. In order to permit observations within a dark body cavity, an intracavitary observation site of interest has to be illuminated with light which is supplied from outside. Therefore, an illumination means is usually provided alongside an endoscopic observation means or optical image pickup system at the distal end of the insertion instrument. The illumination means includes a light guide in the form of a bundle of fine fiber optics as a light transmission means, and an illumination lens is located in front of a light emitting end of the light guide. On the other hand, the optical image pickup system includes an objective lens to form an image of an intracavitary site under observation. Located at the focus of the objective lens is either an image pickup end of an image guide consisting of a bundle of fiber optics, or a solid-state image sensor device like CCD which functions to convert an optical image into electric signals. In the former case, that is, in the case of the so-called optical endoscope, images of an intracavitary site under observation are viewed through an eyepiece which is connected to the head assembly. In the latter case, that is, in the case of the so-called electronic endoscope, electric signals from the solid-state image sensor are sent to a video processor which generates video signals to display video images of the site under observation on a monitor screen.
In an endoscopic examination of an intracavitary site, for example, of a site within the stomach, it may become necessary to bring the endoscopic observation means to a position close to a particular gastric wall portion for closer examination after observing the site from a relatively distant position. At the time of an examination from an extremely close position, the focus of the objective lens of the endoscopic observation means needs to be adjusted correctly. In this regard, for example, Japanese Patent Publication 61-53698 discloses a focus adjusting mechanism for an endoscopic observation system. This focus adjusting mechanism makes it possible to adjust the focus of an objective lens to an intracavitary wall at an extremely small distance less than 1 cm, particularly at a distance of 1 mm to 2 mm. Observations from a position of such minimal observation distance can make it possible to examine intracavitary walls very accurately.
However, even if an objective lens is correctly focused on an intracavitary portion of particular interest, the endoscopic observation means cannot necessarily make accurate observation of the intracavitary portion. As mentioned hereinbefore, an intracavitary sites which is in darkness needs to be illuminated by the use of a light guide and an illumination lens, and the site under observation as a whole should be illuminated as uniformly as possible in terms of illuminance level. This is because existence of dark spots or dark shaded portions in part of the site under observation are hindrous to accurate endoscopic examinations. On the contrary, in case the illuminance level is extremely high in certain localities as compared with the remainder of an intracavitary site under observation, a high contrast between bright and shady portions in the entire areas of the site under observation can also make it difficult to examine the site precisely. Above all, in the case of an electronic endoscope using a solid-state image sensor device, overly illuminated bright portions can cause saturation to corresponding part of the image sensor pixels and result in images with blooming and smear. In addition, even if saturation of pixels does not take place, irregular variations in illuminance level will lower the dynamic range to make it difficult to produce images of good quality.
In this connection, for example, Japanese Laid-Open Patent Specification 6-169879 discloses an endoscopic illumination means which is arranged to suppress irregular variations in illuminance level across an observation area of an intracavitary site. According to this prior art, illumination means are located at equidistant positions on the opposite side of an observation means at the distal end of an endoscopic insertion instrument to project illumination light rays of substantially equal volumes from the two separate illuminating means. In this case, in order to concentrate the illumination light rays toward an intracavitary area under observation, the two illuminating means are located as close to the endoscopic observation means as possible within a range in which an optical image pickup portion of the observation means is free influences of harmful right rays.
The endoscopic insertion instrument to be introduced into a body cavity should be constructed as small as possible in diameter for easy and smooth passage through a path of insertion and for lessening pains on the part of the patient. Nevertheless, in addition to one endoscopic image pickup means and two illuminating means, the endoscopic insertion instrument is required to have a biopsy channel for insertion of a biopsy or surgical instrument or a passage for pumping a cleaning liquid to an observation window of the optical image pickup portion. Therefore, due to limitations in space, the two illumination means are inevitably required to have an extremely small aperture diameter and virtually take the shape of a point source of light. However, illumination light can be diffused by fitting an illumination lens in an illumination window face to face with a light emitting end of a light guide. By so doing, an intracavitary site under endoscopic observation can be irradiated with substantially uniformly in illuminance as long as the intracavitary site is located at a certain distance from the respective illumination means. However, the illumination lens are incapable of diffusing illumination light over a subject at an extremely short observation distance. Especially, in case the endoscopic observation means is located at a distance shorter than several millimeters, more specifically, at a very close distance of 1 mm to 2 mm, the illuminance distribution characteristics is such that the illuminance level is conspicuously hihg at a center position of the illumination window but abruptly drops toward peripheral portions thereof.
Therefore, the illumination system of the above-mentioned prior art, which has a couple of illumination means located on the opposite sides of an endoscopic observation window, is advantageous when concentrating illumination light on an examination site from a certain distance but has a drawback that the difference in illuminance level between center and peripheral areas of an imaging area of the site under examination becomes conspicuously large. Therefore, especially in the case of a solid-state image sensor, blooming and smear occurs to images on display, due to saturation of image sensor pixels in positions which correspond to high illuminance peripheral portions of an imaging area. Besides, due to degradations in dynamic range and S/N ratio, it becomes difficult to display on a monitor screen clear images which are necessary for accurate endoscopic examination or diagnosis.